A wearable device can be worn by a person riding a bicycle. A cyclist may wish to receive a measurement or estimate of their power output and/or calories burned while they ride. One way by which power output can be measured is the installation of a power meter onto a bicycle. However, such meters require installation and may be expensive. Another method for estimating power output is to measure the cyclist's heart rate and generate an estimate of power output based on the heart rate. While inexpensive, heart rate measurements and estimation of cyclist power output based on heart rate can require significant power to operate the sensor. Another approach is to estimate power output based on distance traveled and/or speed at which that distance is traveled. However, estimation based on these quantities can be inaccurate. For example, calculating an estimate of power output based on distance traveled and/or speed at which that distance is traveled will not account for dynamically experienced resistance such as that provided by wind, or by changes in elevation during the ride.
When riding a bicycle, three main factors contribute to total energy expenditure: rolling resistance, contributed by friction of the bicycle tires against the ground; grade, contributed by the force of gravity pulling against the mass of the cyclist and bicycle; and wind resistance or drag, contributed by the force of air drag against the cyclist and bicycle moving through the atmosphere. When riding a bicycle at a constant speed, the total of these three main factors represents the major power output of the cyclist.